Blueberries for home landscapes
Blueberries are increasingly popular fruits with well-documented health benefits. Blueberry plants are also exceptionally handsome bushes worthy of planting in the home landscape. The fruit can be eaten fresh, or frozen for out-of-season use. Plants have a profusion of white blossoms in late spring, and the leaves are glossy green in summer and have outstanding red foliage in autumn.
Blueberry growing presents a challenge for most gardeners because the plants need special growing conditions. They require acidic, well-drained soils, which are not common in most Minnesota landscape situations. When the initial pH is less than 7.0 (slightly acidic), most soils can be amended to make them suitable. In western Minnesota, where the native pH of the soil is greater than 7.0 (basic), amending the soil to a suitable range is very difficult, although construction of a planting area filled with an acidic, well drained soil mixture high in organic matter is possible. Winter hardiness is also a consideration. Production should be successful if cultivar recommendations for your particular area of the state are followed.
Blueberries grow best in a sunny location. Plants will tolerate partial shade, but as shade increases, plants produce fewer blossoms and fruit production declines. Avoid areas surrounded by trees, which provide too much shade, compete with plants for water and nutrients, and interfere with air movement around plants. Poor air movement increases danger of spring frost injury to blossoms and favors disease development.
Blueberry plants grow best in acid soils (pH 4.0 to 5.0) that are well-drained, loose, and high in organic matter. The soil water level should be at least one foot below the soil surface or roots will suffocate. Soil pH can be determined by sending a sample of the soil to the University of Minnesota Soil Testing Facility. Instructions and containers for taking a soil sample can be obtained through your county extension office or the University of Minnesota Soil Testing site. Most garden soils in Minnesota have pH readings above those that are optimum for blueberries (most soils are too basic).
Blueberry plants are long-lived (30 to 50 years or perhaps even longer), so considerable time and effort in preparing the planting site is a wise investment. Soils not within the range of pH acceptability for blueberry plant growth must be prepared BEFORE planting. If the pH is too high, the growth of the plant is slowed and the foliage turns yellow. If the pH is too high for an extended period of time, the plants will die. When several plants are to be grown together, more satisfactory results will be obtained if an entire bed is prepared rather than digging holes for individual plants.
If the pH of the soil is between 5.5 and 7.0, and the texture is sandy to sandy loam, the addition of acid peat is all that will be needed to prepare the soil. Mix 4 to 6 inches of acid peat into the top 6 to 8 inches of soil. In addition to acidifying the soil, the peat increases the soil organic matter content.
Different sulfur compounds can be used to acidify the soil as well. For 50 cubic feet of sandy soil (the amount of soil in a space 10 feet by 10 feet by 6 inches), use one to two pounds of elemental sulfur to reduce the pH one point. You will need to use three to six pounds to get the same effect in loam soils. Elemental sulfur takes at least one year to adjust the pH.
Iron sulfate reacts much faster than elemental sulfur (less than one month); however, the cost is greater. Multiply the rate of elemental sulfur needed by six to determine the required amount of iron sulfate.
Aluminum sulfate is not recommended, although it can acidify soil, because high rates of this compound can be toxic to roots.
Soils with a pH greater than 7.0 will require higher rates of acidifying amendments and are not recommended for growing blueberries. In addition to an acid soil, blueberry plants require a soil that is well aerated and has a high water-holding capacity. Most garden soil is not good blueberry soil, so modification of the soil is frequently necessary.
To grow blueberries where soil is poorly drained and/or too basic to be acidified adequately, prepare a raised planting area. To accommodate two plants, create a raised planting bed 15 inches deep by 24 inches wide by 48 inches long. Fill with a soil mixture of 4 bushels well-rotted sawdust, leaf mold, or peat; 2 bushels loam soil; and 2 cups wettable sulfur. As this soil settles and decomposes over the years, you will need to continue adding sulfur, soil and peat to the planting bed. Continue to have your soil's pH tested every year or two, and amend as needed.
To modify soil that is too dry and sandy, there is no need to create a raised bed; instead, make a hole in the ground of the above dimensions and fill it with the soil mixture.
The University of Minnesota fruit breeding program has released six cultivars suitable for planting in Minnesota. The Michigan State University cultivar 'Northland' is also a good choice for Minnesota gardeners. The following table gives the characteristics of these seven cultivars. Planting at least two varieties is recommended, as more berries of larger size will be produced if flowers are fertilized with pollen from another variety. Bumblebees and other native insects are enthusiastic pollinators of blueberries; the more insects working the plants, the more fruit you will harvest.
Plant young blueberry bushes in late April or early May, spacing them 3 to 4 feet apart. Dig the holes large enough to accommodate all the roots and deep enough so you can cover the uppermost roots with 3 to 4 inches of soil. Pack the soil firmly around the roots, then mulch the planting with 2 to 4 inches of sawdust, peat moss, or chopped straw. Surface mulch helps maintain uniform soil moisture and good soil structure, and it reduces soil temperature in the summer. Replenish the mulch as needed. Water the planting frequently enough to keep the soil moist but not saturated throughout the life of the planting. Drip irrigation or soaker hoses would be useful in larger plantings.
Care of the planting
In the first two years, remove flowers in the spring to encourage vegetative growth. Encouragement of vegetative growth is essential for healthy plants in the following years. Production of flowers and fruits deters growth when plants are too small or weak. A good-sized, healthy canopy is needed to support the fruit. Even healthy, vigorous blueberry plants are slow-growing compared to many other plants; it may be eight or even ten years before full size is reached.
The need for fertilizer will be indicated by plant growth and foliage color. Generally one application of an acid-producing fertilizer each year will be sufficient. Do not fertilize after the blooming period; late fertilizing will encourage late growth in the fall which, in turn, can cause winter injury. The nitrogen used should be in the ammonium form (ammonium sulfate) rather than the nitrate form (calcium nitrate). Blueberries are in the same plant family as azaleas and rhododendrons (Ericaceae), and an azalea fertilizer formulated for acid-loving plants works well for a small planting.
Follow the manufacturer's instructions as to amount and method of application. The objectives of pruning are to remove dead and diseased wood, shape the bush, maintain an adequate number of vigorous main stems to prevent overbearing, and to stimulate new shoot growth. Pruning new bushes is needed only to remove any dead or dying parts of branches. After the fifth year, prune the bushes annually in the early spring, just before growth starts.
Fruit is produced on one-year-old wood. The largest berries are produced on the most vigorous wood, so a good supply of strong, one-year-old wood is desirable. Excessive pruning should be avoided because it greatly reduces the crop for that year. Keep the bush fairly open by cutting out any weak, old stems that no longer produce strong young wood. Remove these older stems at ground level. Keep four to six of the vigorous older stems and one to two strong new shoots per mature bush. The new shoots will eventually replace the older stems.
Blueberry plants tend to overproduce. Often, if all the flowers are left to develop into fruit, the berries will be small and late ripening, and plants will have little new growth. To avoid this, remove most of the thin, weak branches that have many flower clusters and few leaves. This type of pruning can be delayed until the extent of flowering is determined. Try to have a good balance between berry production and growth of vigorous new shoots.
Blueberries are very attractive to birds. Birds can eat the entire crop of a small planting if it is not protected. Covering the individual bushes or the entire planting with netting supported by a light frame is the best protection. Secure the netting so there is no place for the birds to enter. The fruit ripens over a three week period, and you will have to remove the netting to harvest. The netting should not shade the plants or they will not flower well the following year. Be sure to completely remove the netting after harvest.
Young branches of blueberry bushes are attractive to rabbits. Most of the damage from rabbits occurs during the winter when other food is scarce. If rabbits are a problem, enclose the planting with a fine chicken-wire fence. The fence must be high enough so the rabbits cannot get over the top when the snow is deep.
Insects and diseases are not likely to cause problems with most plantings. Careful pruning will help prevent disease infection. Prune out and destroy any part of the plant that is dead or dying. Examine the plants for cankers, that first appear as small, reddish, discolored areas on the stems. As the affected areas enlarge, the margins remain reddish and the bark in the central part turns gray and then brown. Cankers occur most frequently close to the ground but may occur higher on the stem. Stems are usually girdled in one season by cankers. Girdled stems die and their brown foliage is quite obvious. Cut out affected parts several inches below the cankered area.
Reviewed 2009 WW-03463